fix editor crash when saving a new, empty course: don't set up fixed

songs with pSong == NULL.

It'd be better to allow it, and remember song entries in courses that don't
exist, so they can be displayed as "missing song" in the editor, and so editing
a course doesn't erase all entries that are missing; but I don't want to go into
that right now.

stepmania/src/EditCoursesMenu.cpp

@@ -408,7 +408,11 @@ void EditCoursesMenu::OnRowValueChanged( Row row )
 		pEntry = GetSelectedEntry();
 		if( pEntry == NULL )
 		{
-			pCourse->m_entries.push_back( CourseEntry() );
+			CourseEntry ce;
+			const vector<Song*> &apSongs = SONGMAN->GetAllSongs();
+			ASSERT( !apSongs.empty() );
+			ce.pSong = apSongs[0];
+			pCourse->m_entries.push_back( ce );
 			pEntry = GetSelectedEntry();
 		}
 		// fall through